The invention relates to a pressure modulator for a vehicle hydraulic brake system having wheel brakes which are connected via a brake circuit to a brake operator through the pressure modulator and wherein an anti-locking braking control modifies the operation of the brake operation by causing the pressure modulator to reduce the brake pressure applied to the wheel brakes in response to an anti-locking signal from the anti-locking brake control. The pressure modulator comprises: a housing; a moveable modulator piston; a control pressure space; and an annular modulation chamber; an auxiliary pressure source; a sump; and a control valve for alternatively connecting the auxiliary pressure source and the sump to the control pressure space. Connection of auxiliary pressure to the control pressure space by the control valve, causes the modulator piston to reduce the size of annular modulation chamber to a minimum volume, while connection to the sump enlarges the size of annular modulation chamber to a maximum volume.
A pressure modulator of this general type appears in applicant's German Patent Application p. 36 37 781.3-21 as an element in an anti-lock wheel braking system. This pressure modulator has a modulation chamber as a portion of the main brake line of the brake circuit subjectable to the anti-lock control and is delimited by one side of a smaller piston step of a modulator step piston. The larger piston step delimits one side of a control-pressure space of an auxiliary-pressure source to cause the modulator step piston to be forced, counter to the effect of a powerful restoring spring, into a basic position corresponding to a minimum volume of the modulation chamber. In order to control the pressure reduction phases of this anti-lock control, the control-pressure space is connected to a pressureless tank of the auxiliary-pressure source as a result of the changeover of an ABS control valve. The modulator spring displaces the modulator piston with the effect of an increase in volume of the modulation chamber to obtain the desired reduction of brake pressure. A disadvantage of this construction is that the modulator piston, because of its relatively large mass, influences the response behavior of the control to a considerable extent. If a modulator spring with a high restoring force is chosen, so that a higher acceleration of the modulator piston can be achieved, then either an auxiliary-pressure source working at a very high pressure level has to be used, or a very large cross-sectional area has to be chosen for the control-pressure space, and this in turn necessarily entails an increase in the mass of the modulator piston.
The best possible compromise between the spring force and the piston mass must also involve restrictions as regards the response behavior of the control.
The object of the invention is therefore, to improve a pressure modulator of the general type mentioned in the introduction, to effect a faster response behavior of the anti-lock control.
According to the invention, this object is achieved by having the modulation chamber defined by two axial moveable side walls. One side wall being a flange of a first control piston element of a multi-part piston and the other being a piston step of a second annular piston element which surrounds a portion of the first central piston element. The multi-part piston is located in a bore step with a diameter larger than a diameter of the modulation chamber and the two piston elements each have a piston flange which defines moveable side walls of a pressure control space. High pressure led into the control space forces the central piston element against a bias spring to define a minimum volume of the modulation chamber.
As a result of the two-part design provided for the modulator piston, the piston elements move in opposition to a bias spring in a pressure-reducing phase of the anti-lock control, to force an "acceleration" of the response behavior of the anti-lock control. This "acceleration" is compared with when there is only one piston, the mass of which corresponds to the sum of the masses of the two part pistons. In the pressure modulator according to the invention, the dynamic behavior of the so-called reduced mass two part piston, can be kept clearly lower than the arithmetic sum of their masses and it is this reduced mass that determines acceleration. It is advantageous if the mass distribution between the two piston elements is approximately equal.
This can be obtained by having a piston rod connect a radial flange on a first piston to its piston flange. The piston flange serves as one wall of the control pressure space and the radial flange as one wall of the pressure modulation chamber. This piston rod has an inner portion which is surrounded by the second piston element in a bore of the modulation housing. The bias spring surrounds this inner part and biases the first and second pistons away from each other. The relative diameters of the parts starting at the smallest one is: the piston rod, the inner part, the radial flange, the second piston and the two piston flanges that define the control pressure space.
It is also an advantage if there is a widening opening in the second piston adjacent the piston, which leads, via a passageway in the second piston, to another widening and then to a sump. In this manner a drain for fluid by-passing the piston seals is obtained so that brake fluid and control fluid will not be contaminated.
It is also desirable to have an automatic valve to shut-off the pressure modulator from the brake operator unit when the anti-lock control responds. This can be obtained with a valve ball held in a closed position by a spring and opened by a tappet connected to the second annular piston so as to open shortly after the second piston begins to move. The stroke of the inner piston is desirably at least twice the stroke of the outer piston, so that an early shut-off is obtained by having the outer piston actuate the tappet.
A safety valve is held in closed position by the control pressure of the auxiliary pressure source and opens in response to a drop in pressure to ensure that the brake system remains operational in the event of a failure of the auxiliary pressure source.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.